balloon: fix stack-like behavior for balloons

We should always remove the last balloon we've created. There are two main
reasons for that:

1) The older balloons are likely to be already in more tenured generations,
and will move less, whereas younger balloons could be in younger generations.
So by removing them like a stack, we'll avoid needless moves

2) The probe, when inserted, should stay for as long as we can.

That was always the intended behavior but I made the small mistake of inserting
them in the wrong order. Fix that.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>